The present invention relates to an automatic transmission system of an automotive vehicle etc. having an oil cooler to cool transmission oil. The present invention also relates to a hydraulic control device and method for an automatic transmission.
Japanese Laid-Open Patent Publication No. 2007-64268 disclose one type of automatic transmission system, which includes a transmission having a plurality of transmission elements such as gear trains, multiple disc clutches/brakes, belt drives etc. and a hydraulic control device having a manual valve, a valve actuator such as a motor, a shift position sensor and a control unit that controls the valve actuator to actuate the manual valve such that the manual valve switches a supply of hydraulic fluid to hydraulic actuators of the transmission elements based on a detection signal from the shift position sensor. This configuration makes it possible to secure assured shift control of the transmission according to driver's shift lever operation and provide flexibility in vehicle interior/body layout design.
The transmission elements have sliding portions that require to be lubricated with lubricating oil so as to ensure smooth operations and prevent heat generations for improvement in transmission durability. As the lubricating oil increases in temperature by absorption of heat from the transmission elements and thereby decreases in lubrication performance during use, the automatic transmission system is equipped with an oil cooler to cool the lubricating oil and maintain the lubrication performance of the lubricating oil.
However, the viscosity of the lubricating oil increases as the lubricating oil gets cooled down. When the temperature of the lubricating oil becomes too low, a required amount of lubricating oil cannot be supplied to the lubrication-requiring portions of the transmission elements due to viscous drag. This results in deterioration of the transmission durability. In particular, the length of passage of the lubricating oil to the lubrication-requiring transmission portions increases with the flow of the lubricating oil through the oil cooler so that the amount of lubricating oil supplied to the lubrication-requiring transmission portions becomes reduced with increase in flow resistance.
In view of this problem, Japanese Laid-Open Patent Publication No. 2002-266993 proposes that the automatic transmission system has a hydraulic circuit equipped with a cooler bypass valve to bypass the oil cooler and supply the lubricating oil to the lubrication-requiring transmission portions without passing the lubricating oil through the oil cooler upon judging that the viscosity of the lubricating oil becomes high e.g. when the transmission line pressure exceeds a reference value.